1. Field of the Invention
The present invention relates to a process for producing high toughness, high strength steel having excellent resistance to stress corrosion cracking in a stress-corrosive environment such as seawater or salt water. 2. Description of the Prior Art
The growing energy demand of recent years and the need to secure stable supplies of energy have led to a rapid heightening of interest in the development of seabed resources and geological surveys of the seabed. In turn, this has also stimulated the construction of marine structures and seabed research vessels related to such ocean development, and to the construction of seabed bases for oil production and other such purposes.
With these structures needing to be free from distortion, failure, and so forth caused by waves and water pressure, ensuring higher levels of safety is an urgent task. As such, it is required that materials used for such structures have high weldability, high strength, and high toughness, and that they also have high resistance to stress corrosion cracking under service environment conditions such as seawater.
To meet the need for safer, more reliable steel, high strength, low alloy, Ni-containing steels and processes for producing such steels have been developed.
In Japanese Unexamined Patent Publication No. 61-127815, a steel slab is heated at a very low temperature, 900.degree. C. to 1000.degree. C., and is then subjected to low temperature hot rolling and direct quenching, followed by tempering, with the result that the effective grain size is defined to provide a high toughness steel with a higher brittle crack arresting capability than conventional steels.
In Japanese Unexamined Patent Publication No. 59-100214, uniform mechanical properties are imparted to a steel plate by suppressing fluctuation along the length by simultaneously cooling the entire steel plate and by suppressing fluctuation in the thickness direction by reducing the water flow density to minimize the difference in cooling rate between the surface and the interior of the steel plate.
However, none of these disclosures consider stress corrosion in an environment in which there is contact with salt water, such as in the case of marine structures, and thus cannot be considered to be entirely safe for marine use.
Japanese Unexamined Patent Publication No. 61-272316 discloses a process for producing steel having good resistance to stress corrosion cracking in seawater, wherein Ni-containing steel with added Nb and reduced amounts of the impurity elements P, N, and O is hot-rolled and then subjected to direct quenching and tempering.
U.S. Patent Application Ser. No. 120,315/87 discloses improving the resistance to stress corrosion cracking of welded portions by reducing the carbon in a Ni-Mo steel and making up for the drop in strength caused by the lower carbon by utilizing controlled rolling, direct quenching and tempering.
To study stress corrosion cracking in high strength steels, use is made of the linear fracture mechanics mode theory, in which the stress intensity factor (K value) is applied to quantize the fracture behavior of cracks or defects inherent in the material in a corrosive environment. For this, a pre-cracked specimen is subjected to a stress corrosion cracking test under the conditions of the service environment to establish a severe condition at the notch root and accelerate the onset of delayed failure.
For this, a set of constant-load tests at various K levels is performed under the above conditions to obtain the critical K.sub.ISCC value (Mode I fracture occurring under plain strain condition), a constant K value at or below which failure does not take place. This K.sub.ISCC value is utilized to evaluate the resistance to stress corrosion cracking.